Process for packaging radioactive wastes in the form of synthetic rock

Abstract

The present invention relates to a process for packaging radioactive wastes, in which the following successive steps are carried out: a/ radioactive wastes, the solids content of which comprises at least 90% of compounds selected from CaCO 2 , Fe 2 O 3 , SiO 2 , Al 2 O 3 and B 2 O 3 , are supplemented, so as to achieve a target composition of said supplemented wastes after calcination, and b/ said supplemented radioactive wastes are melted and c/ said melt is poured into a container, so as to obtain, after cooling, a product comprising a vitreous or vitro-crystalline synthetic rock, having said target composition, characterized in that said target composition corresponds to the following definition, in a CaO, SiO 2 and X 2 O 3 ternary system, in which X 2 O 3 is a trivalent oxide or a mixture of trivalent oxides selected from Al 2 O 3 , Fe 2 O 3 or B 2 O 3 , P C and P S being the mass percentages of CaO and SiO 2 : P C is from 35 to 60%, and P S is from 10 to 45%.

The invention claimed is: 
     
       1. A method for packaging radioactive wastes, wherein the following successive steps are carried out:
 a/ radioactive wastes are treated for which the composition of the dry extract after calcination at 950° C., called starting composition hereafter, comprises at least 90% of compounds selected from CaO, Fe 2 O 3 , SiO 2 , Al 2 O 3  and B 2 O 3 , and the composition of said wastes is supplemented so as to attain a target composition of said supplemented wastes after calcination, and 
 b/ said supplemented radioactive wastes are melted and 
 c/ said melt is cast in a container, so as to obtain after cooling, a monolith product comprising a synthetic either glassy or vitro-crystalline rock, having said target composition, 
 wherein said starting composition and said target composition meet the following definitions, in a ternary (CaO, SiO 2  and X 2 O 3 ) system, wherein X 2 O 3  is a trivalent oxide or a mixture of trivalent oxides selected from Al 2 O 3 , Fe 2 O 3  et B 2 O 3 : 
 for said starting composition: 
 P C  and P X  are less than 90%, and 
 P S  is less than 75%, and 
 for said target composition: 
 P C  is from 35 to 60%, and 
 P S  is from 10 to 45%, 
 with, in both cases: 
 P C +P S +P X =100%, and 
 P X =P A +P H +P B , with 
 P C =[M C /(M C +M S +M A +0.28M H +2M B )]×100%, and 
 P S =[M S /(M C +M S +M A +0.28M H +2M B )]×100%, and 
 P A =[M A /(M C +M S +M A +0.28M H +2M B )]×100%, and 
 P H =[0.28M H /(M C +M S +M A +0.28M H +2M B )]×100%, and 
 P B =[2M B /(M C +M S +M A +0.28M H +2M B )]×100%, and 
 P i  and M i , with i=C, S, A, H or B, are the mass percentages (P i ) and the masses (M i ) of CaO (i=C), SiO 2  (i=S), Al 2 O 3  (i=A), Fe 2 O 3  (i=H) et B 2 O 3  (i=B) respectively. 
 
     
     
       2. The method according to  claim 1 , wherein:
 in step b/, the radioactive wastes are heated in a crucible and are melted at a temperature of 1,250 to 1,650° C., and 
 in step c/, said melt is cast into a container, with a capacity of at least 200 L, so as to form said monolith and said thereby melt packaged is cooled, without any annealing, down to room temperature, within a period of less than 15 days. 
 
     
     
       3. The method according to  claim 2 , wherein in step c/, the cooling step is carried out in two steps, namely:
 c.1/ the cooling rate of said container filled with said melt is limited in the cooling phase between 1,250 and 1,000° C., to a cooling rate comprised between 50° C./h and 250° C./h, and 
 c.2/ the cooling of said container is then completed from 1,000° C. down to room temperature, without any limitation of the cooling rate, by placing said container in open air or quenching it in cold water until its temperature is lowered down to room temperature. 
 
     
     
       4. The method according to  claim 1 , wherein said radioactive wastes comprise of limestone soil, concrete rubble, sludges from nuclear power plants, concentrates from evaporators of nuclear power plants, sand, and/or ashes of incinerated radioactive waste. 
     
     
       5. The method according to  claim 1 , wherein said initial radioactive wastes have an starting composition meeting the following definition in the ternary CaO/SiO 2 /X 2 O 3  system, wherein X 2 O 3  and P i  have the meanings given in  claim 1 , with:
 Pc and P X  are less than 75% and Ps is less than 60%. 
 
     
     
       6. The method according to  claim 1 , wherein said target composition corresponds to the following mass percentages:
 P C  comprised between 40 and 50%, and 
 P S  comprised between 20 and 40%. 
 
     
     
       7. The method according to  claim 1 , wherein in step a/, the following steps are carried out, wherein:
 a.1/ a limestone soil and/or concrete rubble are treated for which said starting composition meets the following definition in a ternary CaO/SiO 2 /X 2 O 3  system, wherein X 2 O 3  and P i  have the meanings given in  claim 1 : 
 P C  is comprised between 30 and 80%, and 
 P x  is less than 20%, and 
 a.2/5 to 50% of X 2 O 3  selected from Al 2 O 3 , Fe 2 O 3  and B 2 O 3  are added. 
 
     
     
       8. The method according to  claim 7 , wherein in step a.2/, a B 2 O 3  addition of less than 10% of the mass of radioactive wastes to be treated is carried out and/or P B  is less than 15% in said target composition of the obtained monolith. 
     
     
       9. The method according to  claim 8 , wherein in step a.2/:
 the B 2 O 3  addition is less than 5% of the mass of radioactive wastes to be treated in step a.1/ and/or P B  is less than 7% in said target composition of the obtained monolith, and 
 the addition of Al 2 O 3  and Fe 2 O 3  is greater than 10% of the mass of radioactive wastes to be treated in step a.1/, and/or P X  is greater than 15% in said target composition of the obtained monolith. 
 
     
     
       10. The method according to  claim 1 , wherein the mixing of radioactive wastes of different compositions is achieved in order to obtain said target compositions, without adding non-radioactive trivalent oxide(s) selected from Al 2 O 3 , Fe 2 O 3  and B 2 O 3 . 
     
     
       11. The method according to  claim 5 , wherein the method is proceeded with the mixing of:
 1/ said limestone soil and/or said concrete rubble of said following starting compositions: 
 P C  comprised between 50 and 80% and P X  less than 20%, and 
 2/ a sludge of radioactive wastes, preferably a sludge from a nuclear power station, of said following starting composition: 
 P X  comprised between 10 and 70% and P C  less than 50%. 
 
     
     
       12. The method according to  claim 7 , wherein in step a/, the following steps are carried out wherein:
 a.1/ limestone soil and/or concrete rubble are treated, for which said starting composition meets the following definition in the ternary CaO/SiO 3 /X 2 O 3  system: 
 P C  is comprised between 50 and 80%, and 
 P S  is comprised between 20 and 50%, and 
 P X  is less than or equal to 20%, X 2 O 3  being a trivalent oxide or a mixture of trivalent oxides selected from Al 2 O 3  and Fe 2 O 3 , and P B =0, and 
 a.2/ the additive containing X 2 O 3  is added in order to attain said following monolith target composition in the CaO/SiO 3 /X 2 O 3  ternary system: 
 P C  is comprised between 35 and 55%, and 
 P S  is comprised between 15 and 40%, and 
 P X  is comprised between 10 and 45%. 
 
     
     
       13. The method according to  claim 1 , wherein the large particles with a size of more than 1 cm, are extracted from the limestone soils so that the composition of the fine portion approaches the target composition, so that it attains said target composition. 
     
     
       14. The method according to  claim 1 , wherein:
 in step b/, said additional radioactive wastes are melted by Joule effect heating, by means of graphite electrodes immersed in the material of radioactive wastes to be treated, and 
 in step c/, a biphasic product is obtained, comprising a synthetic rock matrix having said target composition wherein in X 2 O 3  is selected from Al 2 O 3  and B 2 O 3 , said matrix incorporating cast iron inclusions. 
 
     
     
       15. The method according  claim 2 , wherein:
 before introducing said radioactive wastes into said crucible, milling of said radioactive wastes is carried out in order to obtain a grain size of less than 5 cm of at least one portion of the particles which it contains, and 
 the fumes released during the melting of the radioactive wastes are cooled to less than 200° C. and the gaseous radio-elements such as cesium, which they contain, are trapped in a particle filter.